1. Field of the Invention
The present invention relates to a directivity controllable array antenna that is used in communication equipment of a microwave band, a millimeter wave band or the like and provided with a variable phase shifter or a variable reactance circuit, and to a radio communication apparatus using the same. More specifically the invention relates to an array antenna and a radio communication apparatus using the same in which for a variable phase shifter or variable reactance circuit is used a variable capacitance capacitor having a dielectric layer whose dielectric constant changes in accordance with an applied voltage, which array antenna or radio communication apparatus is capable of changing a phase shifting amount or a reactance value by changing the capacitance and making the directivity of the array antenna variable, and in particular, which has excellent characteristics such as high power handling capability, low distortion and low loss, and is of low-price and easily configured.
2. Description of the Related Art
On communication equipment of a microwave band, a millimeter wave band or the like, demands for making radio communications high speed and large capacity, improving radio communication quality and realizing high speed mobile communications and so on have increased year by year. However, depending on states of a radio communication environment, multipath and a Doppler shift may cause deterioration of the radio communication quality. Moreover, because of spread and increase of mobile phones and the like in recent years, it has become necessary to increase the number of users who can simultaneously communicate.
As a solution for solving these problems, for the purpose of improving the radio communication quality and enhancing the use efficiency of a certain limited frequency so that a lot of users can use the frequency simultaneously by effective use, an adaptive array antenna technique of adaptively controlling a directivity of an antenna receives attention, and is enthusiastically examined today.
As a solution for making it possible to adaptively control a directivity of an array antenna, an array antenna that makes it possible to adaptively control the directivity by using a variable capacitance diode or a voltage-control-type dielectric varactor as a reflective termination portion and combining with a rat race coupler or the like, or by using a variable phase shifting circuit in which the voltage-control-type dielectric varactor is placed in a radial stub extending from a microstrip line, is proposed (refer to Japanese Unexamined Patent Publication JP-A 2002-528899, for example). This voltage-control-type dielectric varactor is composed of a substrate, a controllable ferroelectric layer, and first and second electrodes. The substrate has a first dielectric constant and has a substantially flat surface. The controllable ferroelectric layer has a second dielectric constant larger than the first dielectric constant and locates on the substantially flat surface of the substrate. The first and second electrodes locate on a surface of the controllable ferroelectric layer opposite to the substantially flat surface of the substrate. The first and second electrodes are separated so as to form a gap therebetween. The voltage-control-type dielectric varactor is equivalent to a variable capacitance capacitor.
However, in the related array antenna having the variable phase shifting circuit using the variable capacitance diode as a variable phase shifter, a loss at high frequencies of the variable capacitance diode is large, so that there is a problem such that an electric power loss in the variable phase shifting circuit becomes large, and consequently, a loss of the array antenna becomes large.
Further, the related array antenna having the variable phase shifting circuit using the variable capacitance diode has a problem such that it can be used only in a receiver, a reception circuit and the like that handle small electric power, because high power handling capability of the variable capacitance diode is low, and a distortion of signals resulting from nonlinearity of a capacitance is large. In other words, the related array antenna has a problem such that it cannot be used in a transmitter and a transmission circuit that handle large electric power.
Additionally, in the related array antenna having the variable phase shifting circuits using the variable capacitance diodes, as shown by an equivalent circuit diagram of an example of the variable phase shifting circuit in FIG. 14, bias signals are supplied from a bias terminal V via a bias supply circuit G to variable capacitance diodes 301 and 302, so that the variable phase shifting circuit needs the independent bias supply circuit G composed of choke coils L1 and L2. Therefore, it is necessary to design the bias supply circuit G, it is necessary to take time for regulation thereof, and moreover, the variable phase shifting circuit and the bias supply circuit G are configured separately, so that there is a problem such that the area of the circuits becomes large and the array antenna apparatus becomes large in size as a whole. In respect of the need for the bias supply circuit G, the same problem is caused in the array antenna having the related variable phase shifting circuits even if the variable capacitance diodes are replaced with variable capacitance capacitors.
Besides, in the array antenna having the variable phase shifting circuits using the variable capacitance diodes, the variable capacitance diodes 301 and 302 have polarities with respect to applied voltages, so that there is a problem such that it is necessary to pay attention to the polarities not only at the time of designing but also packaging and it takes time to mount.
Furthermore, in the related variable phase shifting circuit using the voltage-control-type dielectric varactor as proposed in JP-A 2002-528899, the voltage-control-type dielectric varactor corresponding to the variable capacitance capacitor causes a capacitance variation even by a high-frequency voltage, so that there is a problem such that, in a case where the high-frequency voltage is high, distortion characteristics such as a waveform distortion and an intermodulation distortion of the variable phase shifting circuit become large. Moreover, in order to make the distortion characteristics small, it is necessary to lower a high-frequency electric field strength of the variable capacitance capacitor and decrease the capacitance variation by the high-frequency voltage, and it is effective therefor to broaden a gap of a capacitance forming portion. However, there is a problem such that since a direct current electric field strength also lowers when the gap of the capacitance forming portion is broadened, a capacitance change ratio also lowers and a variable width of a phase shifting amount of the variable phase shifting circuit decreases.
Still further, since an electric current easily flows to the variable capacitance capacitor in the case of high frequency signals, when the variable capacitance capacitor is used at high frequencies, the variable capacitance capacitor generates heat because of a loss resistance and breaks down, so that there is a problem such that high power handling capability of the variable phase shifting circuit is low. It is also effective for the problem of the high power handling capability to broaden the gap of the capacitance forming portion (increase the thickness of a dielectric layer) and reduce a heat generation amount per unit volume. However, when the gap of the capacitance forming portion is broadened (the thickness of the dielectric layer is increased), the direct current electric field strength is also reduced, so that there is a problem such that the capacitance change ratio also lowers and the variable width of the phase shifting amount of the variable phase shifting circuit decreases. In the case of applying the variable phase shifting circuit to the array antenna, it is necessary to connect a variable phase shifting circuit having a large component size for each antenna element of the array antenna, so that there is a problem such that the array antenna becomes large in size.
Further, as a directional antenna in which a radiation element to be fed and a reflector or director serving as a parasitic element are placed in specified positions and combined, a Yagi-Uda antenna is typical, and an array antenna apparatus in which a plurality of parasitic radiation elements excited by mutual coupling are placed around a radiation element to be excited and variable reactance elements are loaded on the parasitic radiation elements and which can control a directivity by changing reactance values of the variable reactance elements, is proposed (refer to Japanese Unexamined Patent Publication JP-A 2002-299952, for example). The array antenna apparatus is provided with the driven element, and a plurality of variable reactance circuits disposed on each of the parasitic elements, and the array antenna is configured so as to cause the plurality of parasitic elements to operate as the directors or reflectors and change the directivity of the array antenna by changing the reactance values of each of the variable reactance circuits. The variable reactance circuits use a variable capacitance diode serving as the variable reactance element, and the variable capacitance diode corresponds to a variable capacitance capacitor.
Furthermore, at least one pair of variable reactance elements such that the variable capacitance diodes are connected in the opposite direction to each other are used in the variable reactance circuit, and by restraining a nonlinear distortion such as the second harmonic of the variable capacitance diode and connecting a plurality of pairs of variable reactance elements in parallel, an array antenna for large electric power that can withstand a large electric current is realized.
However, in the related directivity controllable array antenna, the variable reactance circuit uses at least the one pair of variable reactance elements such that the variable capacitance diodes are connected in the opposite direction to each other. In the related array antenna having the variable reactance circuit, a loss at high frequencies of the variable capacitance diode is large, so that there is a problem such that a loss of the variable reactance circuit becomes large, and consequently, a loss of the array antenna becomes large.
Further, in the variable reactance circuit, in order to withstand a large electric current, it is necessary to connect the plurality of pairs of variable reactance circuits in parallel, and it is necessary to increase the pairs of variable reactance circuits by connecting a plurality of pairs in parallel as a handled electric current becomes large. In this case, there is a problem such that a loss of the variable reactance circuits further increases.
Furthermore, the variable reactance circuit loaded on the parasitic element needs at least the one pair of variable reactance elements such that the variable capacitance diodes are connected in the opposite direction to each other, and the array antenna apparatus has a problem such that the number of steps for mounting the variable reactance elements increases, the cost of components increases, and because of an increase of a component count, an influence of variations in the components increases.
Still further, in order to get ready for a large electric current, it is necessary to increase the number of the variable reactance elements connected in parallel, so that such a problem is caused in this case that the number of the steps for mounting the variable reactance elements further increases, the cost of the components increases, and the influence of the variations in the components due to the increase of the component count further increases.
Additionally, in the array antenna having the variable reactance circuits using the variable capacitance diodes, as shown by an equivalent circuit diagram of the variable reactance circuit loaded on a dipole-type parasitic element in FIG. 15, bias signals are supplied from bias terminals Vc− and Vc+ via bias supply circuits G1 and G2 to a pair of variable capacitance diodes 301 and 302, so that the variable reactance circuit needs the independent bias supply circuits G1 and G2 composed of resistors R1, R2 and R3. Therefore, it is necessary to design the bias supply circuits G1 and G2, it is necessary to take time for regulation thereof, and moreover, the variable reactance circuit and the bias supply circuits G1 and G2 are configured separately, so that there is a problem such that the area of the circuits becomes large and the array antenna apparatus becomes large in size as a whole. In respect of the need for the bias supply circuits, the same problem is caused in the array antenna having the related variable reactance circuits even if the variable capacitance diodes are replaced with variable capacitance capacitors.
Besides, in the array antenna having the variable reactance circuits using the variable capacitance diodes, the variable capacitance diodes 301 and 302 have polarities with respect to applied voltages, so that there is a problem such that it is necessary to pay attention to the polarities not only at the time of designing but also packaging and it takes time to package.